PROJECTSUMMARY/ABSTRACT Chronic cocaine exposure causes maladaptive neuroplasticity in brain reward circuitry that leads to compulsive drug use, loss of control over intake, and vulnerability to relapse, even after protracted abstinence.Initialdrugrewardismediatedbythenucleusaccumbensintheventralstriatum,andearly goal-?directed drug intake is dependent on this structure. As drug use transitions from goal-?directed to compulsive,thereisacorrespondinganatomicaltransitionfromdependenceontheventralstriatumto dorsal striatum habit circuitry. MicroRNAs (miRNA) are small (~21-?23 nucleotide) non-?coding RNA sequences that regulate mRNA stability and translation by complementary binding to the 3? UTR of the mRNA. Each miRNA can regulate the binding of hundreds or even thousands of mRNA. Our lab has previously identified that miR-?212 regulates cocaine intake under extended access conditions, but not during restricted access. Specifically, overexpressing miR-?212 in the dorsal striatum decreases cocaine intakeduring6-?hourself-?administrationsessions,whereasusingaknockingdownmiR-?212inthisregion augmentscocaineintakeinrats.TheprecisecellularmechanismofactionofmiR-?212isnotknown.The dorsal striatum is primarily composed of medium spiny neurons (MSNs; 95% of cells), which can be further broken down by their receptor composition and anatomical projections. Dopamine D1 receptor (DRD1) expressing neurons form a direct pathway that innervates the substantia nigra (striatonigral pathway),andneuronsexpressingtheD2receptorandtheadenosineA2Areceptor(A2AR)projecttothe pallidum(striatopallidalpathway).Thesecellpopulationshaveoppositecontributionstotasksinvolving movement,andmorenotablyreward,suchascocaineconditionedplacepreference.Thisproposalseeks to examine the contributions of miR-?212 specifically in each of these two cell populations in order to elucidate its mechanism of action in regulating cocaine seeking. To do this, in aim 1 I will use novel genetically modified mice to create conditional knockouts of miR-?212 in either DRD1-?expressing MSNs, or in A2AR-?expressing MSNs. These mice will be tested under either restricted or extended access to cocaine,andthecontributionofmiR-?212duringeachofthesephaseswillbeassessed.Inaim2,Iwilluse anovelhighthroughputscreeningfollowingcrosslinkingimmunoprecipitation(HITS-?CLIP)techniqueto examinemRNAtargetsofmiR-?212specificallyinstriatonigralvsstriatopallidalneurons.Together,these aims will provide me with excellent training in mouse genetics and behavioral models, as well as advanced RNA biochemistry and large high throughput screening data sets, including bioinformatics analysisofsuchdatasets.